Wafer bonding is a technology used in micro-electronics fabrication, in which a first substrate carrying first devices on its surface is brought into contact with second devices on a surface of a second substrate for fabricating an electronic circuit. Typically, the contact is arranged in such a way that signals can be transported from at least one first device on the first substrate to at least one second device on the second substrate and vice versa. The signals to be transported between the first device and the second device may be either electronic, photonic, fluidic, gaseous, magnetic, or sonic, or may relate to any other conceivable type of signal. The reason for wafer bonding is a cost of reduction for production and/or packaging and also an overall tendency of product shrinkage.
An example where wafer bonding is useful in reducing costs is when Si-based first devices need to be combined with second devices based on another substrate material such as a III-V or II-VI semiconductor substrate. Manufacturing of both types of devices on a single substrate may be virtually impossible due to the incompatibility of the constituent materials.
Another example is an electronic circuit made out of a first and second substrate where for reason of complexity, devices need to be fabricated on separate wafers (even though both substrates may be based on a similar substrate type).
In cases where an electronic circuit or “chip” needs to combine two or more devices not manufactured on a single substrate, wafer bonding can provide a way to combine those devices into a single package with a desired (electronic) function. On each device, area are reserved for providing one or more contact pads. A contact pad is intended for providing a connection between one of the devices and a similar contact pad on the other of the devices. Typically, the location of (a) contact pad(s) within the device area is defined during the design of the device or the electronic circuit of which the device is a part. The contact pads on devices to be bonded must have a sufficient alignment and overlap (i.e., a coincidence in their respective lateral positions within the device area) that a functional contact between the first and second devices is achieved: i.e. the electronic circuit formed from the devices is actually functional.
Note that for circuits where signals other than electric signals need to be exchanged between the first and second devices, instead of contact pads contact openings (for channels guiding the signals) may be provided. In that case the bonding operation must take care of aligning the contact openings on first and second devices with respect to each other to have functional contact between the first and second devices.
Conventional wafer bonding machines may provide a wafer bonding capability but with a limited accuracy of about 100 micrometers. In order to compensate the limitation of the accuracy by overlap, it is desirable that the contacts of both first and second devices to each other be relatively oversized in comparison to the contact sizes obtainable by lithographic processing (typically 0.25 μm or less).
Consequently, it is not possible to bond wafers manufactured with current lithographic technology in an efficient manner, since the needed “slack” of the contact pads on both devices on both substrates to have an sufficient overlap which allows successful bonding (i.e., to obtain an electronic circuit that can function) consumes a lot of the available wafer estate.
Moreover, conventional wafer bonding machines provide an alignment of first and second substrates in a merely mechanical way without providing a reference to the product location: the mechanical alignment between substrates is assumed to coincide with the alignment for devices between substrates. However, such an approach is not cost-effective since the yield may be relatively low.
Nevertheless, for device structures of relatively low density and relatively large feature size, such trial and error method could provide a sufficient bonding quality. For modern device structures with higher density this method may no longer be usable.